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There is JSON in the response, but what they actually return is JavaScript that initializes a variable with JSON. Their new V2 API returns "true" JSON but it requires signing up for an API Key or OAuth.
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Alex AngasSep 28 '13 at 6:46

The old API supports JSONP in The Usual Way -- pass ?callback=foo and you get foo({...}) instead of var tumblr_api_read={...}. The API docs don't mention CORS support so I strongly suspect most users would be loading the content via JSONP anyway.
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CodererSep 28 '13 at 11:50

OOps, sorry I missed you were looking for hierarchical data, this is a flat source. I'm was having a hard time looking for public feeds without key registration needs and found this interesting to share.
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BraulioNov 22 '13 at 18:08